Roaming occurs in a wireless network when a mobile device, such as a wireless phone, personal digital assistant (PDA), or other mobile device connects to a network other than its home network. Many wireless-networks providers have contracted with each other to allow its customers to use the services of a second network for a fee (though often transparent and unbeknownst to the mobile subscriber). By such agreements, the wireless-network providers pay each other for their subscribers' usage of other wireless networks.
By negotiating agreements with other wireless-networks providers, a given provider is able to offer continuity of service over a larger area without having to build out its own wireless network. But nevertheless, for a variety of reasons, a home carrier often would like for its users to use its network.